In the semiconductor industry, there is an ever-increasing desire to increase memory density and performance. These goals are often achieved by scaling dynamic random access memory (DRAM) devices to smaller dimensions and operating voltages.
Vertical DRAM devices use a trench to form both a signal storage node and a signal transfer device. Vertical DRAM devices have been proposed to increase memory density because they decouple the length of the vertical signal transfer device channel from the minimum feature size. This configuration allows longer channel lengths without a proportional decrease in memory density. Channel length may then be properly scaled relative to gate oxide thickness and relative to junction depth to reduce channel doping, minimize junction leakage, and increase retention times.
FIG. 1 shows a partial cross-sectional view of a vertical DRAM device or cell 100 formed in a substrate 101 (typically P-silicon). The DRAM cell 100 is formed using a trench (DT or deep trench) having a sidewall 122. The DRAM cell 100 includes a signal storage node (partially shown) 102 which includes a storage node conductor 104 (typically N+ polysilicon) and a collar oxide 106. The signal transfer device of the DRAM cell 100 includes a first diffusion region 108, a second diffusion region 110(typically N+ silicon), a channel region 112, a gate insulator 114, and a gate conductor 116 (typically N+ polysilicon).
The gate conductor 116 is coupled to the wordline 118. The wordline 118 comprises an N+ polysilicon lower layer 118A, a WSi.sub.x middle layer 118B, and a nitride cap layer 118C. The second diffusion region 110 is covered by a nitride layer 120. The storage node conductor 104 is covered by a trench-top oxide (TTO) 123. A shallow trench isolation (STI) region 128 is formed to provide isolation for DRAM device 100.
The trench sidewall 122 of the DRAM cell 100 is a distance W from the sidewall 124 of the trench of an adjacent DRAM cell. For DRAM cells 100 occupying a 5F.sup.2 surface area of the substrate 101, where F is the minimum feature size, the distance W between adjacent trench sidewalls may be 2F. With a trench-to-trench distance W of 2F, a wordline 118 can overlap past the sidewall 122 of the trench by a distance of 0.5F. This configuration allows adequate overlap of the gate conductor 116 by wordline 118 even in the worst case of misalignment when DT and wordline bias are under control. DRAM cell density on a wafer may be increased by decreasing the trench-to-trench spacing W. As trench-to-trench spacing W is reduced below 2F, the probability that the wordline conductor will not overlap the trench edge increases because the layed out overlap of the wordline to the trench is reduced below 0.5F while the alignment tolerance remains constant.
DRAM cell 100 in FIG. 1 has a wordline 118 which does not completely overlap the trench sidewall 122. This incomplete overlap causes the etch used to form the wordline 118 to cut into the underlying gate conductor 116 as illustrated by the gate conductor over-etch 105. Over-etch 105 may result in damage to the gate insulator 114 and a failure of the gate conductor 116 to overlap the second diffusion region 110.
To overcome the shortcomings of conventional DRAM devices, a new vertical DRAM device is provided. An object of the present invention is to provide a vertical DRAM device that has a wordline conductor self-aligned to the sidewall of the trench. A related object is to provide a process of manufacturing such a vertical DRAM device. Another object is to provide a pair of vertical DRAM devices each having a respective wordline and each formed using a respective trench in which the distance between the respective trenches equals the distance between respective wordlines. It is still another object to provide a vertical DRAM device having a wordline positioned above the surface of the substrate.